Anaerobic digestion (AD) involves the breakdown of complex organic molecules through acetate- and H2/CO2-mediated methanogenesis to methane (CH4)-containing biogas. The advantages over conventional aerobic treatment systems, of anaerobic waste mineralisation include: a reduction in excess sludge production and the release of methane (CH4), a readily usable fuel, which may be harnessed for external energy uses. Practically all full-scale AD facilities, however, are operated under mesophilic (25-45° C.) conditions (Letting a et al., 1999), the maintenance of which incurs considerable financial costs, using a significant fraction of the biogas energy. The most significant amount of parasitic energy is required to bring the temperature of many wastewaters up to the optimal mesophilic range, since the overwhelming majority of discharges are released for disposal and/or treatment at sub-ambient temperatures. If the need for heating could be removed, AD would be much more economically attractive. In addition, and importantly, AD is currently not widely applied for the treatment of dilute wastewaters, such as sewage—because the energy required to heat the digester often exceeds that recoverable from the biogas. This is a key drawback to conventional AD, which has meant that energy-intensive aerobic technologies, such as Activated Sludge, have been the technology of choice for treatment of municipal and dilute industrial wastewaters for decades.
AD, or methanogenesis, at low temperatures has been described in a variety of natural habitats, however, including tundra and permafrost soils and the sediments of deep-lake ecosystems (Nozhevnikova, 2000), which suggested that low-temperature AD could be a viable target for a novel eco-technology. Psychrophilic, or low-temperature (<20° C.) AD would indeed, if proven feasible, present a highly attractive alternative to conventional operations, offering a low-cost, low-technology methodology for the treatment of many municipal and industrial effluents (Letting a et al., 2001). The application of low-temperature digestion has clear economic benefits in this scenario and this sustainable approach promises to satisfy the socio-economic criteria for the implementation of modern remediation systems on a truly global basis. In addition to this, the possibility of anaerobic mineralisation of environmentally persistent, pharmaceutical or xenobiotic wastewaters (Bioremediation) represents an exciting new commercial application of low-temperature AD.
In addition to carbon removal, wastewater treatment increasingly requires the removal and recovery of phosphate. Globally, phosphate is a diminishing resource, vital for the production of agricultural fertilizers, and there are increasing commercial and legislative drivers requiring its recovery from wastewater. A number of mechanisms for the attenuation of phosphate are known in the art, including the attenuation of phosphate by sorbent materials. Under alkaline conditions, soluble phosphate ions react with calcium to form a sequence of Ca—P phases, for example. Under acidic conditions, phosphate anions (H2PO4−, HPO42−) may react with dissolved Fe3+, Al3+ and Mn3+ to form insoluble hydroxy-phosphate precipitates or may be fixed by insoluble oxides of Fe, Mn, and Al. Anion exchange, which is a pH dependent mechanism may also occur where hydroxyl anions are released and replaced by phosphate ions. With increasing acidity surface charge tends towards a greater positive charge, while increasing pH produces a negatively charged surface. The process involves non-specific electrostatic forces that render the phosphate anions readily exchangeable. The phosphate ion may also replace a structural hydroxyl to form an inner-sphere complex with the oxide surface in ligand exchange. This reaction is also favoured by low pH values. This reaction also binds the phosphate too tightly to allow its readily replacement by other anions. The binding forces involved are covalent bonding, ionic bonding or combination of the two making the recoverability of phosphate very low—a drawback to the use of these materials.
It is therefore an object of the present invention to provide commercially viable low temperature or psychrophilic anaerobic digestion for methane production and phosphate removal from wastewater or other effluents.